Avoid unnecessary manifest generation in env_spec.
Avoid unnecessary manifest generation in env_spec.

The default manifest can use the pre-generated manifest_with_defaults
which saves an extra manifest generation run.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf